A typical apparatus for the production of spunbond yarn includes a spinneret fed by extruders, a cooling chamber where the filaments undergo a first partial cooling, a stretching unit and a deposition unit to deposit the stretched filaments onto a mobile support where the desired non-woven fabric is formed. The mobile support is generally an air permeable belt onto which the filaments are deposited and retained by air blown by special means; downstream of the point of deposition of the filaments on the belt there is a pair of rollers to calender the layer of deposited filaments.
All these modules (extrusion, cooling, stretching and depositing) are very important to achieve a good final product.
Among these modules, particular importance have the stretching area and that of deposition of the filaments on the mobile support. In fact, the count of the filament depends on the extent of the stretching of the same filament and such stretching is also controlled by the stretching airflows in the stretching area and area of deposition of the stretched filament.
The patent application US 2003/0178742, in the name of Reifenhauser, describes an apparatus in which the cooling area is connected to the stretching area without an opening for the supply of air. The stretching area is provided with two diffusers, each of which has a section reduction to produce a Venturi effect and therefore an acceleration of the air. Between the first and second diffuser there is an inlet for secondary air that is the only inlet other than that for the cooling air in the cooling chamber. The area of suction includes three regions of aspiration located in succession in the area of deposition of the stretched filaments on the collecting belt; such areas are provided with distinct means of aspiration for each region or, alternatively, a single aspiration fan connected to the three areas through ducts and valves.
Patent application US 2003/0161904, in the name of Reifenhauser, describes a system for depositing filaments on a mobile collecting belt by aspiration, according to which there are at least two aspiration areas under the mobile collecting belt controlled independently of each other, one such area being the principal, located corresponding to the area of greatest deposit of the filaments. The speed of aspiration in the principal area is significantly higher than that of the other aspiration areas (second and third area of aspiration).
EP-A-1630265 discloses an apparatus for the continuous production of a nonwoven web from filaments made from two thermoplastic synthetic materials (FIGS. 5 and 6), i.e. multi-component filaments. The apparatus comprises a spinning nozzle, a cooling chamber, a stretching unit and a collecting device for depositing filaments onto the nonwoven web. The cooling chamber is divided into two or more cooling chamber sections to supply filaments with process air at different temperatures. The collecting device comprises a mobile belt, which is gas permeable, at least one aspirator and a plurality of suction ducts (referred to as 18, 19 and 20 in FIG. 4) to define a plurality of suction areas on the mobile belt. The suction ducts have different sections to provide different suction forces on the belt. The embodiments discussed above and in general those of the state of the art don't allow the desired uniformity of fiber density to be achieved or maintained along the whole layer of deposited filaments. Excessive variation in the deposition of filaments on the collecting belt translates into a finished fabric whose mechanical characteristics vary from area to area of the same belt, something obviously to be avoided.
EP-A-1630265 teaches the suction force to be adjusted locally by controlling the air speed in each suction duct located under the collecting belt, with the aim of compensating uneven deposition of the filaments falling on the belt. This solution still does not avoid uneven deposition of the filaments on the moving belt, and this may cause the non-woven fabric to be formed with random non-uniform portions.
A further problem is the necessity to increase the speed of the mobile support, i.e. the belt, without jeopardizing the uniformity of deposition of the filaments and therefore the composition of the non-woven fabric.
It is an aim of the present invention to solve the aforementioned problems and to provide an apparatus and a process for the production of nonwoven fabrics having uniform filament distribution at higher speed that it is now possible.